When dimming electric lamps, in particular fluorescent lamps having electronic ballasts, an oscillation can typically be formed in the lower region of the dimming range, in particular in the lower third of the dimming range, and this oscillation results owing to the interaction of the lamp characteristic, the resonant circuit and the regulation. This range, which is also referred to as the “frequency reversal” range, occurs to a particularly severe extent during a run up phase of amalgam lamps. At extreme ambient temperatures, such a “frequency reversal” range is also observed in the case of mercury lamps. This oscillation brings about severe modulation of the lamp current at frequencies of from 30 Hz to 10 kHz and cannot usually be recognized as optical instability. The high crest factor of the lamp current which is caused thereby can, however, result in severe damage to the electric lamp and have a life-shortening effect.
In order to avoid operation with a high lamp current crest factor, attempts are made to keep the range of “frequency reversal” small by means of suitably selecting the resonant circuit and the regulation and to reduce the damaging effect of the high crest factor by means of increased lamp filament heating. Furthermore, either the range for the permissible ambient temperature for the dimming operation is restricted or the permissible dimming range at extreme ambient temperatures is reduced. In addition, it is also known in the case of some operating devices, in particular electronic ballasts, for amalgam lamps for the full dimming function to be released only after a predetermined period of time after they have been switched on, in order, as a result, to bridge the run up phase.